Sputter coating is a widely used technique for depositing a thin film of material on a substrate. In a sputtering deposition process ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target of coating material at the cathode by an electric field causing atoms of the target material to be ejected from the target surface. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material.
In operation, when the sputter gas, e.g. argon, is admitted into a coating chamber, a DC voltage applied between the cathode and the anode ionizes the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged cathode. The ions strike the target in front of the cathode with a substantial energy and cause target atoms or atomic clusters to be sputtered from the target. Some of the target particles strike and deposit on the wafer or substrate material to be coated, thereby forming a film.
To attain increased deposition rates and lower operating pressures, magnetically enhanced cathodes are used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate of coating material. Thus, the magnetic field causes the electrons to travel in a closed loop, commonly referred to as a “race track”, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability, thereby leading to a much higher sputtering rate than that obtained without the use of magnetic confinement. Furthermore, the sputtering process can be accomplished at a much lower gas pressure.
Typically a magnetron sputtering system is operated at a pressure of 2*10^−2 Pa-1*10^−1 Pa. during sputtering. To establish this pressure, typically the chamber is pumped down to a pressure of <1*10^−4 Pa. and a controlled flow of a gas, typically Argon (and in case of reactive sputtering Argon and Oxygen or Nitrogen) is fed into the chamber to maintain the desired pressure. In the case of a diode system, i.e. when no magnets are used, a pressure of >2 Pa. is required to be able to ignite and sustain a plasma. High pressure has the disadvantage that the mean-free path is greatly reduced, which causes extensive gas scatter. This results in hazy coatings.
In the magnetron sputtering device the anode provides a charge differential to the negatively charged cathode. This can be provided as simply as an electric charge provided to the chamber walls. However, the sputtered material is also deposited on any surface exposed to the sputtered atoms. If the coating is an electrically insulating material, such as a metal oxide, the build up of the material on other parts of the sputtering apparatus can cause problems. In particular, the build up of an insulating coating on the anode interferes with the ability of the anode to remove electrons from the plasma, as required to maintain the plasma's charge balance. This destabilizes the plasma and interferes with deposition control. Coating build-up will cause the anode location to move to another surface in the system. This instability affects coating quality. Numerous prior art anodes have been proposed to overcome the problems of the anode becoming coated with the coating material. Many prior art anodes function at very high voltages that also increase the problems of arcing, which damages coating quality. A low voltage anode that can provide a stable anode location is desired for ensuring consistent coating quality.
An anode vessel is disclosed in US Publication No. 20060049041 filed Mar. 7, 2005 that can provide a stable anode location at low voltage. The anode comprises an interior surface of a vessel having a single opening in communication with the coating chamber. The interior surface of the vessel is the preferred return path for electrons. The anode vessel is also the source of sputter gas, which passes from an inlet port in the anode vessel through the single opening into the coating chamber. The size of the single opening and its location prevent coating material from building up on the charged interior surface of the anode.
Many optical coatings require the deposition of oxides or other compounds. Such materials are preferably produced in reactive sputter mode where a metallic target is sputtered and oxygen, nitrogen, or another reactive gas is added to the process. The sputtered material and the activated oxygen species arrive simultaneously at the substrate. The optimum flow, of oxygen for example, for the optimum oxygen partial pressure needs to be found. If the oxygen flow is too low, the films are not stoichiometric and have high absorption losses. If it is too high, the target surface becomes more oxidized than necessary preventing operation at the highest possible deposition rate. The sputter rate for a metallic target can be ten times higher than that of a fully oxidized target. The oxidation effectiveness can be increased if the oxygen is activated and directed at the substrates, thus increasing the possible deposition rate. The reactive sputter process is disclosed for oxides. All aspects can similarly be applied to nitrides or other reactive processes.
In order to produce dielectric coatings in a magnetron sputtering device with low or no optical absorption profile, it is necessary to provide an additional activated reactive gas source to provide Oxygen or Nitrogen to create a plasma. Examples of commercially available activated reactive gas sources include a PAS from JDSU, a Taurion source from ProVac, a KRI source from Kaufman & Robinson, an APS source from Leybold. Current activated reactive gas sources are complicated. Some require expensive electronics. Some require filaments that have limited lifetime. These devices are very expensive and can be maintenance intensive.
A Prior Art configuration of the anode vessel 19 and a separate reactive gas source 36 is illustrated in FIGS. 3A and 3B. The cathode 12 is positioned with its center at the central rotational axis C. An anode vessel 19 providing an ionized source of Argon gas is disposed on one side of the cathode 12 and the reactive gas source 36 of ionized Oxygen is disposed on an opposite side of the cathode 12. A substrate 17 rotates about the circumference of the cathode 12 over the reactive gas source 36. In the case of reactive sputtering with a standard cathode 12, a large variation of target wear has been observed which limits the target utilization. The target 14 can be seen in the cross section in FIG. 3B, on the side close to the reactive gas (oxidation) source 36, the target wear is low, due to an increase of target oxidation (poisoning); whereas on the side that is close to the anode 19, the target wear is high, due to an increase in plasma density.
A simpler, less expensive and more reliable source for activated reactive gas in a magnetron sputtering device is highly desirable.
It is also desired to increase the efficiency of oxidation of the deposited film in order to increase the deposition rate for reactive sputtering.
It is also desired to maintain a low temperature process despite increased power input in order to be able to process temperature sensitive materials.